Operations are carried out in oil and gas wells for conveying tools downhole in the well. A wide variety of downhole tools may be supported on a wireline including tools to perform logging, setting, and retrieving operations. The tools typically comprise a combination of different tubular members threaded together to form a working unit that is manipulated from the surface via the wireline. The tools are lowered into and pulled out of the well selectively to perform the desired operation.
With a wireline tool, it is not unusual for the tools to become stuck as they are being retrieved from the well. However, the amount of tension that can be applied to the wireline in freeing the tools is limited by the strength of the wireline itself. Too much tension placed on the wireline causes it to break, typically at the surface. Consequently, tension put on the wireline is typically limited to 50% of the breaking strength of the wireline when new. The wireline and tools must then be recovered from the borehole. Major problems occur if the tools get stuck in the well and the wireline breaks upon pulling on the wireline with too much tension. Breaking the wireline and dropping the wireline in the well greatly complicates the fishing operation to retrieve the tools.
To prevent breaking the wireline when the tools become stuck, wireline tools typically include a cable head that connects the end of the wireline to the top of the tools. A release is also typically provided at the cable head to permit the wireline to be disconnected from the tools such as when the tools become stuck downhole. A wireline release may use a mechanical weak point in the connection between the wireline and the cable head, such as a metal member designed to break upon a predetermined pull on the wireline. The correct conventional mechanical weak point must be calculated and installed prior to running the cable head and tools into the borehole on the wireline. When the tools do become stuck, enough pull is placed on the cable head to release the weak point. A fishing tool is then run downhole on a pipe string, such as drill pipe or tubing, and connects to a fishing neck on the cable head. The fishing tool and pipe string allow a greater amount of pull to be exerted on the tools for extraction from the wellbore. The tools may also comprise a fishing neck for connection by a fishing tool. Various other apparatus and methods are also available for releasing the wireline from the cable head and tools.
Wireline tools can be run downhole using gravity as long as the inclination of the borehole with respect to the vertical does not become so great as to make displacement of the tools using gravity inoperable. For these highly inclined wells, the tools may be conveyed into the borehole using a pipe string with the tool attached to the downhole end of the pipe string. The tools communicate with the surface and are powered by a wireline that enters the inside of the pipe string through a pack-off in a side-entry-sub. When the tool approaches the zone of interest, the side-entry-sub is connected to the pipe string. The tool comprises a male connector portion at the top of the tool and inside the pipe string. A corresponding female connector portion at the end of the wireline is then conveyed down through the tubing and connected to the male connector portion of the tools to form what is called a “wet connect”. The connection is called a wet connect because typically, the female connector portion is pumped down the inside of the pipe string using a fluid such that the connectors are immersed in the pumping fluid when the connection is made. The wireline exits the pipe string through the side-entry-sub and connects with the power and operating systems for the tool on the surface. The exposed portion of the wireline outside the pipe string is located in the annulus between the pipe string and the casing.
With the wet connection made, the side-entry-sub is run further downhole and more pipe sections are added to the pipe string until the tools reach the zone of interest. The side-entry-sub is typically only lowered to the lowermost section of casing so the exposed wireline does not travel into the open borehole. There may also be special cases where the side entry sub exits the casing and enters into the open borehole. In the example of a logging tool, the pipe string and tool are then raised with the logging tool receiving logging information on the formation until the side-entry-sub reaches the surface and must be removed from the pipe string. In some cases, the side-entry-sub must be removed while there is still formation to be logged. As such, the wet-connect must be broken, the side-entry-sub must be removed from the top of the pipe string, the pipe repositioned, the side-entry-sub must then be reinstalled, and the wet connection must be formed again by pumping the wireline to the tool at the end of the pipe string. This process is repeated until the entire formation zone of interest is logged.